(a) Field
The present invention relates to optical pressure sensors, and more specifically to optical pressure sensors based on the Fabry-Perot interferometer.
The subject matter disclosed generally relates to the design of a new optical pressure sensor undergoing less mechanical stresses having detrimental impact on the performance of the sensor.
(b) Related Prior Art
There is a variety of existing optical pressure sensors based on the Fabry-Perot interferometer. These types of sensors often differ by their optical assembling and mechanical mounting methods. For example, U.S. Pat. No. 7,689,071 by Belleville teaches that the construction of a Fabry-Perot pressure sensor comprises a bi-directional optical fiber that guides light waves toward a Fabry-Perot-based optical pressure cell made from a glass substrate (the sensor body) and a silicon deflecting diaphragm. A first reflective mirror is deposited within a recessed cavity performed on the top surface of the glass substrate. A deformable silicon diaphragm is bonded or welded to the glass substrate to form a second mirror and also to tightly seal the recessed cavity. The two mirrors, spaced by a distance given by the depth of the recessed cavity constitute a Fabry-Perot interferometer. The depth of the recessed cavity, called the cavity length of the Fabry-Perot interferometer, varies as a function of the differential pressure that may exist between the inside and the outside of the sealed cavity. This optical pressure cell is mounted at the end of an optical fiber within a receiving cavity created in the sensor body of the cell. The receiving cavity is filled with adhesive to secure the cell in place and to seal the whole assembly. One major drawback of this method is the use of adhesive for sealing and bonding. Such sealing and bonding method can only work in low differential pressure environments. Another drawback of this method is that the sensor body needs to be thin (of the order of 200 microns) because no lenses are used to bring the light waves to the pressure cell. Thin optical components are more prone to optical distortion, which may affect the pressure measurement. Another drawback of this method is that the sealing body is applied against the sensor body. Therefore the resulting mechanical force which is required for leak-tight sealing are transferred to the sensor body creating internal stresses in the sensor body. This again can lead to optical distortion of the pressure cell and then affect the accuracy of the pressure measurements.
U.S. Pat. No. 5,128,537 by Beat Halg teaches a different arrangement where the cavity can be put at a given pressure by using a secondary pressure port. This design has similar aforementioned drawbacks with the sealing body applied against the sensor body and the requirement of a thin sensor body.
Alternatively, in U.S. Pat. No. 4,933,545 by Saaski et al., optical lenses are used to bring the light waves to the optical pressure cell. In that case, the sensor body can be made more robust by increasing its thickness. Nevertheless, this design is also plagued with a similar aforementioned drawback because of the sealing body being applied against the sensor body. Another drawback of this arrangement is that a large differential pressure may exist between the front side and the back side of the optical pressure cell. This may result in the bending of the cell which in turn affects the pressure measurements.
Another arrangement is disclosed in U.S. Pat. No. 7,614,308 by Berner et al. A support disc is added to the optical pressure cell and the sealing body is applied against the support disc. Although the forces exerted by the sealing body are applied on the support disc, it is not possible to avoid some coupling of these forces to the optical pressure cell knowing that the support disc must be tightly sealed against the cell. Also large pressure differences that may exist between the front side and back side of this optical assembly (support disc with pressure cell) can induce the bending thereof. This in turn can result in the aforementioned optical distortion in the cell which affects the pressure measurements.
These aforementioned optical pressure cell arrangements are all exposed in one way or the other to mechanical stresses. This can have a detrimental impact on the performances of the sensor. For example, it is well known in the art that these kinds of stresses tend to relieve themselves unpredictably with time, with temperature change or under vibration and/or shock. These stress relief effects are the main source of unwanted drift of the pressure sensor. The pressure cell is also affected by the bending forces which, for instance, can severely affect the linearity of the sensor response.
There is therefore a need for a new optical pressure sensor less exposed to mechanical stresses and accordingly being designed to avoid the detrimental impacts associated to these stresses.